Field of the Invention
The invention relates to a grid structure for an elongate fuel assembly with rods disposed parallel and next to one another, between which a stream of coolant is conducted through the fuel assembly. The invention also relates to such a fuel assembly with a spacer and/or a mixing grid with such a grid structure.
Published European Application No. 0 364 623 which discloses such a fuel assembly for a pressurized water reactor and German Published, Non-Prosecuted Application DE 15 64 697 A1 which uses spacers with mixing vanes, are described below in the description of the drawings.
It has also already been proposed to place such grid structures with mixing vanes even in planes of the pressurized water reactor which in fact do not require any spacer for reasons of stability, in order to reinforce the lateral mixing-through of the coolant and to permit a radial temperature compensation. In such a device, the webs, in the case of spacers have holding assemblies (for example in the form of dimples and springs) for fixing the lateral position of the fuel rods, if anything also include safety stops, in order to prevent the rods from striking against the mixing vanes and damaging them in the event of vibration.
Such mixing vanes have also already been proposed in the case of boiling water reactors, although mixing-through over a number of fuel assemblies is not possible therein due to the fuel channels and their walls. In that case, however, a swirl produced in flow subchannels leads to vortices in the axial flow, which especially in the upper part of the fuel assembly, where the coolant is in the form of a liquid/vapor mixture, leads to a wetting of the fuel rods by liquid droplets contained in the mixture. As a result, the occurrence of hot spots is reduced, which particularly at high power outputs could lead to a drying out of the liquid film flowing along the fuel rods and to a worsening of the heat transfer ("dry out").
In that case, it is often necessary for the same flow resistance for the through-flowing coolant to occur in each case at all of the fuel assemblies of a reactor core, since differences in the flow resistance also cause corresponding differences in the flow distribution and cooling effect at the individual fuel assemblies.
Thus, for example, if after an operating cycle some of the old fuel assemblies are replaced by fuel assemblies with corresponding fittings to improve the temperature distribution, the flow resistance with respect to the neighboring fuel assemblies is worsened specifically because of the grid structures being employed, and consequently the coolant throughput is reduced in the "improved" fuel assemblies. That often has the effect of compensating or overcompensating for the success which is to be achieved.
Such a troublesome pressure loss on one hand occurs at a constriction of the flow cross section and an adjoining widening to the original size, which are caused by the webs and their crossing points as well as the spacing assemblies and mixing vanes disposed thereon (compression and expansion). On the other hand, the vortices and turbulences produced by the mixing vanes also result in further pressure losses.
Therefore, it has already been proposed to compensate for such pressure loss by reducing the fuel rod diameter, for example from 9.5 mm to 9.14 mm. In the case of fuel rod production that means that machines, tools and storage spaces have to be additionally set up for the new type of fuel rod. However, due to the reduction of the fuel rod diameter, the fuel content and the possible burn-up in the fuel assemblies is also reduced.
It may, however, also be an object from the outset to distribute the envisaged fuel content over fuel rods which are as thin as possible, with a correspondingly greater number of them having to be chosen. In that case, however, the flow resistance increases with the number of fuel rods and requires a fluidically favorable construction of the fuel assembly internals.
In U.S. Pat. No. 3,928,131 a spacer with longitudinal webs and with transverse webs which cross the longitudinal webs approximately perpendicularly is described. The crossing points all lie in one plane and the longitudinal and transverse webs are all of the same width at the crossing points. The webs in that case have dimples for supporting the fuel rods approximately in the center between two crossing points and on a level which lies approximately in the center between the upper edge and lower edge of the crossing points. In order to increase the flexibility of that grid structure, the transverse webs and longitudinal webs are significantly narrowed in the vicinity of the dimples, producing zigzag-shaped edges, on which the period of the zigzag-shaped course or progression corresponds to the distance between the center axes of neighboring fuel rods, that is the width of a flow subchannel.
Published French Application No. 2 578 348 shows spacers on which upper edges of approximately perpendicularly crossing webs run partially in zigzag form with a period corresponding to the half-period of the zigzag always above a level which corresponds approximately to the center plane of the spacer and on which the crossing points also lie, and the corresponding lower edges always run below that level.